The present invention relates to a method of processing perfluorocarbon and an apparatus therefor, and more particularly, the invention relates to a preferable method for use in processing perfluorocarbon (hereinafter, called PFC) contained in an exhaust gas from a semiconductor manufacturing plant, and an apparatus therefor.
In a known semiconductor manufacturing process, various uses are made of PFC gases, which are harmless to a human being, are nonexplosive, and are easy to handle, such as CF4 and the like, which are used as an etchant in a dry etching process, and C2F6 and the like, which are used as a cleaning gas in a CVD process. These PFC gases are ionized by a plasma discharge of a high voltage after being introduced into an etching apparatus or a CVD apparatus, and are used to perform etching or cleaning of wafers in an active radical state.
However, the amount of PFC gas actually consumed in the etching or the cleaning is several % to tens % by volume.
The rest of the PFC gas is exhausted outside the system in an unreacted state.
TABLE 1Properties of gasConsumeWarm.Life.2)Toxic.in JapanNoPFC gascoeff.1)(year)react.3)(t/year)Main use1CF44)6,30050,000low-300('94)Etching gas: 75%toxic.394('95)CVD cleaningnon-gas: 25%flam.5)2CF66)12,50010,000low- 4('94)P-CVD cleaningtoxic,200('95)gasnon-flam.5)3NF37)9,720179toxic 25('94)CVD cleaning 39('95)gas: 92%IC-Etching gas:8%4CHF38)12,100250— 55('94)dry etching5C4F89)8,7003,200——etching6C3F810)7,0002,600——P-CVD cleaninggas7SF611)24,9003,200— 26('94)dry etching: 90%CVD cleaninggas: 10%Remarks:1)Warming-up coefficient2)Life time in atmosphere3)Toxicity and reactivity4)Flon 145)Non-flammable6)Flon 1167)Nitrogen trifluoride8)Flon 239)Flon C 31810)Flon 21811)Sulfur hexafluoride
Because a fluorine atom has a small atomic radius and a strong bonding force, PFC, a compound of fluorine atoms, has stable characteristics. PFC includes flon, such as FC (fluorocarbon) and HFC (hydrofluorocarbon), which do not include chlorine, and perfluoride compounds, such as nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6). Main materials of PFC, and their characteristics and main use, are indicated in Table 1.
PFC exists stably in the atmosphere for a long time, and, because it does not contain chlorine, its molecular structure is compact, and its bonding force is strong. For instance, the life of CF4 is as long as 50,000 years, the life of C2F6 is 10,000 years, and the life of SF6 is 3,200 years. However, PFC has a large warming-up coefficient. In comparison with CO2, CF4 is 6,500 times, C2F6 is 9,200 times, and SF6 is 23,900 times. Therefore, although a smaller amount of PFC is released than CO2, which is required to be decreased since it is a cause of warming-up of the earth, it is anticipated that the release of PFC will certainly be restricted in the near future. In this case, a countermeasure against release of the exhaust gas from semiconductor manufacturing plants, which is the source of a majority of the PFC being released, will become an important consideration.
For instance, in an etching step performed in a semiconductor manufacturing plant, a PFC gas for etching is supplied into a chamber. A part of the PFC gas is converted to highly corrosive fluorine atoms by applying a plasma thereto. The fluorine atoms perform an etching of silicone wafers. The exhaust gas from the chamber is pumped out continuously by a vacuum pump. In order to prevent corrosion by the acidic gas, purging of the exhaust gas with nitrogen gas is performed. The exhaust gas contains nitrogen in the amount of 99% and PFC in a residual amount of 1%, which has not been used for the etching. The exhaust gas pumped out by the vacuum pump is conducted to an acid removing apparatus, through the duct for removing the acidic gas, and is released into the atmosphere in a state in which it contains the PFC.
In the semiconductor manufacturing plants, a reagent method and a combustion method have been used practically as a method of decomposition of PFC. The former is a method wherein fluorine is chemically fixed at approximately 400-900° C. by using a special reagent. In accordance with this method, exhaust gas processing is not necessary, because no acidic gas is generated by the decomposition. The latter is a method wherein the PFC gas is conducted to a combustor and is decomposed thermally in a flame of at least 1,000° C. generated by combustion of LPG and propane gas.
In accordance with the above reagent method, the reagent which is reacted chemically with the PFC cannot be re-used, and the expensive reagent, which is consumed in the reaction as a consumable article, is required to be supplied frequently. Therefore, the operation cost is 10 to 20 times in comparison with that of the combustion method. Furthermore, because an amount of the reagent equivalent to the amount of the PFC to be processed is necessary, practical equipment for performing the reagent method requires a large area, such as approximately 3-5 m2.
In accordance with the above combustion method, thermal decomposition is performed at a high temperature, such as at least 1,000° C. for C2F6 and at least 1,100° C. for CF4, and a large amount of thermal energy is required. Furthermore, the combustion method generates NOx and a large amount of CO2 by combustion at a high temperature. Because the PFC is exhausted in a state in which it is diluted with inactive N2 gas, a potential for miss-fire is high, and a sufficient operation control is required.
An application of the combustion method to the semiconductor manufacturing process has been studied. The PFC is exhausted as a mixed gas diluted with N2 gas having a concentration of several %. Accordingly, in the combustion of the mixed gas, a large amount of air for combustion is required in addition to a fuel gas. Consequently, because the amount of gas to be processed is increased, the size of the apparatus is increased, and the area for the apparatus is required to be as large as approximately 0.7-5 m2.
For instance, when C2F6 is contained in the amount of 1% in an exhaust gas exhausted at 100 liter/min. from a semiconductor manufacturing process, the necessary amount of LPG to make the thermal decomposition temperature at least 1,000° C. is 10 liter/min., and the necessary amount of air is approximately 400 liter/min. with an excessive ratio of 1.5. The total amount of the exhaust gas after the combustion becomes approximately 500 liter/min., because oxygen in the air is consumed and CO2 is generated at a rate of 30 liter/min. The total amount of the exhaust gas is increased almost 5 times that of the exhaust gas exhausted from the semiconductor manufacturing process. The typical semiconductor manufacturing plant has a large restriction on space, because the plant must be provided with clean rooms. Accordingly, it is difficult to provide the necessary area for installing a new exhaust gas processing apparatus in a previously built semiconductor manufacturing plant.
On the other hand, a catalytic method, wherein PFC is decomposed at approximately 400° C., has been applied to CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon), which have similar chemical compositions with PFC and an ozone destruction effect. Because CFC and HFC contain chlorine atoms having a large atomic radius in their compositions, the molecular structures composed by bonding fluorine atoms and hydrogen atoms having a small atomic radius are distorted. Therefore, CFC and HFC can be decomposed at a relatively low temperature.
A method of decomposing CFC (or HFC) using a catalyst was disclosed in JP-A-9-880 (1997). In accordance with this method, a mixed gas of heated air, which is made up of a carrier gas, steam and CFC, is conducted to a catalyst layer. The temperature of the catalyst layer is approximately 430° C., because CFC has a low decomposition temperature. The exhaust gas containing decomposed gases exhausted out of the catalyst layer is cooled rapidly with cooling water, in order to prevent generation of dioxine.